The present invention relates to a process for producing an olefin polymer, according to which process a lumped matter having a sheet-like form can hardly be formed on an inner wall surface of a gas phase polymerization reactor.
In recent years, olefin polymers have generally been produced according to a gas phase polymerization using a fluidized bed reactor equipped with a plate having many pores (hereinafter referred to as xe2x80x9cgas distribution platedxe2x80x9d) at the bottom of the reactor. Such a gas phase polymerization has a problem that powder particles, particularly small particles, which form a fluidized bed, adheres to an inner wall surface of the reactor owing to static electricity generated by friction between particles in the fluidized bed reactor, or between particles and the inner wall surface of the reactor, thereby causing insufficient removal of heat of the polymerization reaction, and as a result, a lumped matter having a sheet-like form is easily formed on the inner wall surface of the reactor. When the lumped matter having a sheet-like form accumulates to increase its weight, the lumped matter peels off from the wall, and as a result, there are risks of closing an outlet of the olefin polymer and filling the pores of the gas distribution plate.
As a method for solving the above-mentioned problem, there are known:
(1) a method as disclosed in, for example, JP-A 10-60019 wherein a static electricity-removing agent such as an amine-containing antistatic agent is added to the fluidized bed, thereby inhibiting generation of the static electricity; and
(2) a method as disclosed in, for example. JP-A 8-169915, wherein a low-frequency high-pressure sonic wave is generated in the reactor, thereby forcibly peeling off the lumped matter having a sheet-like form adhering on the inner wall surface of the reactor.
However, these methods cannot satisfactorily solve the above-mentioned problem.
Accordingly, at present, it is obliged to stop the operation periodically to remove the lumped matter having a sheet-like form adhering on the inner wall surface of the reactor for avoiding the above-mentioned problem.
Considering said gas phase polymerization from a viewpoint of an olefin polymerization catalyst, an olefin polymer produced using a homogeneous solid catalyst, which catalyst comprises a combination of a transition metal compound (such as a metallocene complex and a non-metallocene complex) with an organoaluminum compound (such as an aluminoxane) and/or a boron compound (such as tri(n-butyl)ammonium tetraxis(pentafluoro)borate), and which catalyst has become used in recent years, has a lower melting point than that of an olefin polymer produced using a heterogeneous solid catalyst, which catalyst comprises a combination of a transition metal catalyst component (such as a titanium compound) with an organoaluminum compound (such as trlethylaluminum). Therefore, the former gas phase polymerization using a homogeneous solid catalyst is remarkably liable to cause the above-mentioned problem.
It is an object of the present invention to provide a process for producing an olefin polymer with no problem as mentioned above.
The present inventors have undertaken extensive studies to find a process for producing an olef in polymer with no problem as mentioned above. As a result, it has been found that the above-mentioned problem can be solved by subjecting an olefin to gas phase polymerization in the presence of a homogeneous solid catalyst having a specific particle size, and thereby the present invention has been obtained.
The present invention provides a process for producing an olefin polymer, which comprises a step of polymerizing an olefin in a gas phase in the presence of a homogeneous solid catalyst having a content of particles having a particle size of not more than 180 xcexcm of not more than 15% by weight based on 100% by weight of the total weight of the homogeneous solid catalyst.
A homogeneous solid catalyst for polymerizing an olefin used in the present invention is not particularly limited. As said catalyst, (i) a catalyst comprising a combination of a transition metal compound (such as a metallocene complex and a non-metallocene complex) with an organoaluminum compound (such as an aluminoxane) and/or a boron compound (such as tri(n-butyl)ammonium tetraxis(pentafluoro)borate), and (ii) a catalyst produced by supporting or impregnating the aforementioned compounds with or on a solid particle such as porous silica can be exemplified. Particularly preferred is a catalyst produced by supporting a metallocene complex and methylaluminoxane with porous silica.
The olefin polymer produced by the process in accordance with the present invention means not only an olefin polymer produced by polymerizing an olefin in the presence of the foregoing homogeneous solid catalyst, but also an olefin polymer produced by polymerizing an olefin in the presence of a catalyst, which catalyst is hereinafter referred to as xe2x80x9cpre-polymerization catalystxe2x80x9d, and the pre-polymerization catalyst can be produced by polymerizing a small amount of an olefin in the presence of the foregoing homogeneous solid catalyst. The homogeneous solid catalyst used in the present invention also means said pre-polymerization catalyst.
The homogeneous solid catalyst and pre-polymerization catalyst used in the present invention may be used in combination with a known co-catalyst or a known activity accelerator. As the co-catalyst and the activity accelerator, those disclosed in U.S. Pat. Nos. 4.405,495 and 4,508,842 can be exemplified. Specific examples of the co-catalyst are organoaluminum compounds and specific examples of the activity accelerator are organosilicon compounds.
The catalyst used in the present invention has a low content of fine particles, namely, a catalyst having a content of particles having a particle size of not more than 180 xcexcm of not more than 15% by weight, preferably not more than 10% by weight, and more preferably not more than 5% by weight based on 100% by weight of the total weight of the homogeneous solid catalyst.
A preferred catalyst used in the present invention has a content of particles having a particle size of not more than 180 xcexcm of not more than 15% by weight, and has a content of particles having a particle size of not more than 125 xcexcm of not more than 3% by weight, preferably not more than 1% by weight, and more preferably 0% by weight based on 100% by weight of the total weight of the homogeneous solid catalyst.
The catalyst having the above-defined particle size can be prepared by, for example, (i) classifying a catalyst having a large content of fine particles to decrease contents of particles having a particle size of not more than 180xcexcm and those having a particle size of not more than 125xcexcm, or (ii) using a carrier having a low content of fine particles, with or on which carrier the catalyst components are supported or impregnated.
A gas phase polymerization method used in the present invention is not particularly limited, and may be a conventional one. For example, said olefin polymerization can be carried out using a gas phase fluidized bed reactor in the presence of an effective amount of the catalyst and in the absence of any catalyst poison such as moisture, oxygen or carbon dioxide under conditions of temperature and pressure capable of polymerizing an olefin. Specific methods are disclosed in, for example, U.S. Pat. Nos. 4,482,687, 4,558,790 and 4,994,534.
A gas phase polymerization pressure in the present invention is that in which an olefin can exist as a gas phase in a reactor. The pressure is usually from 0.1 to 5.0 MPa, and preferably from 1.5 to 3.0 MPa. A gas phase polymerization temperature can be suitably selected depending upon conditions such as catalysts used, pressure and a kind of the olefin used, and is usually from 50 to 110xc2x0 C. A gas flow velocity in a reactor during polymerization is usually from 10 to 100 cm/s, and preferably from 20 to 70 cm/s. In the process in accordance with the present invention, additives such as a fluidizing co-agent and a static electricity-removing agent may be used.
As the above-mentioned gas phase fluid bed reactor, there can be exemplified a single reactor and a multi-stage reactor composed of several reactors arranged in series. The reactor""s type such as its shape is not limited. The reactor may be provided with a stirring blade or not.
As an olefin used in the present invention, there can be exemplified ethylene and xcex1-olefins. The xcex1-olefins are, for example, those having 3 to 8 carbon atoms. Specific examples thereof are propylene, 1-butene and 1-hexene.
An olefin polymer obtained in the present invention contains, for example, polyethylene, homopolymers and copolymers of the xcex1-olefin, and copolymers of ethylene and the xcex1-olefin. Specific examples thereof are polypropylene, ethylene-1-butene copolymer and ethylene-1-hexene copolymer.